Answer
$$\frac{dR}{dM}=CM-M^2$$
Work Step by Step
$$R=M^2\Big(\frac{C}{2}-\frac{M}{3}\Big)$$
Find $dR/dM$: $$\frac{dR}{dM}=\frac{d\Bigg(M^2\Big(\frac{C}{2}-\frac{M}{3}\Big)\Bigg)}{dM}$$
We would first apply the Derivative Product Rule: $$\frac{dR}{dM}=(M^2)'\Big(\frac{C}{2}-\frac{M}{3}\Big)+M^2\Big(\frac{C}{2}-\frac{M}{3}\Big)'$$
Here, since $C$ is a constant and only $M$ is the concerned variable, we treat $C/2$ just like any other number.
$$\frac{dR}{dM}=2M\Big(\frac{C}{2}-\frac{M}{3}\Big)+M^2\Big(0-\frac{1}{3}\Big)$$
$$\frac{dR}{dM}=CM-\frac{2M^2}{3}-\frac{M^2}{3}$$
$$\frac{dR}{dM}=CM-\frac{3M^2}{3}=CM-M^2$$
